MINA, South Dakota (CNN) -- Federal investigators will soon hear whatever evidence is stored on the cockpit voice recorder from a runaway Learjet that killed six people, including golf champion Payne Stewart.

The recording device was recovered late Wednesday night from the crash site in a soggy pasture in South Dakota. It will be taken to the National Transportation Safety Board laboratory in Washington where the tapes will be screened for any information.

Bob Benzon, the NTSB's chief investigator, had said early Wednesday that his agency was anxious to analyze the contents of the tape.

"We could get quite a bit of information," said Benzon.

"It picks up ambient noises in the cockpit, switch movements, it picks up any noise, really. We can even analyze the frequency of the engines and determine how fast they were rotating by listening to the cockpit voice recorder."

From miles up in the air, the plane plunged to the ground Monday, burrowing into the field outside the town of Mina after flying on automatic pilot from Florida, nearly 1,500 miles away.

The impact virtually shattered the twin-engine plane, with most of the pieces located in a crater 30 to 40 feet wide and 10 feet deep.

About a quarter of the plane's wreckage has been recovered. Among the parts unearthed by a backhoe Wednesday was the plane's right engine.

"It's virtually intact, heavily damaged of course, but not broken apart," said Benzon. "The left engine was more fragmented into several large sections."

Benson described the recovery operation as a "rough archeological dig," and investigators were uncovering the site in layers, stopping to let the FBI recovery team take care of all human remains.

Crash Investigators want to examine two air valves, one from each engine, that regulated air pressure inside the plane.

One theory is that decompression may have incapacitated the pilots, leaving the jet to fly for hours on automatic pilot until it ran out of gas and crashed.

Benson said his agency has been told that one valve was recently replaced.

"The left-hand modulator valve was changed the Saturday prior to the accident. It's a routine maintenance type thing," said Benson. "It modulates bleed air from the engine that's used for air-conditioning and pressurization of the aircraft."

He said it was too early to make anything of the replacement. The valve is one of two on the airplane and the other would have taken over if the new valve had failed, he said.

Sunjet Aviation, the company that owns the Learjet 35, told investigators that both valves controlling air pressure inside the plane were replaced four years ago in accordance with a Federal Aviation Administration directive.

The 1995 order tells Learjet owners to replace valves "to prevent rapid decompression of the airplane due to cracking and subsequent failure of certain outflow/safety valves."

NTSB Vice Chairman Robert Francis said Tuesday the temperature in the cabin, if the plane did decompress, would have been minus 70 degrees Fahrenheit (minus 56 Celsius) at the 45,000-foot (13,720-meter) altitude it reached, which humans could not survive.

Investigators have said it will probably be several days before experts are able to take a look at seals designed to keep the cabin of the 23-year-old aircraft pressurized, assuming enough of them can be recovered.

Benzon said much of the debris recovered so far consisted of small pieces that have been loaded on several flatbed trucks and transported to a nearby airport hangar for closer examination at another time.

Investigators hope to finish all work in the field by Friday.

Benzon said "several passenger oxygen masks" were retrieved Wednesday. "There is a possibility we may be able to tell whether they had deployed from the ceiling of the aircraft or not, but we're not to that point yet."

"They're activated by pulling -- which breaks a little valve or wire or something up there -- and if all of them that we find happen to have broken wires, then that's a pretty good indication that they were deployed," explained Benzon.

Crash site investigators have also turned up personal effects of the six passengers including a set of golf clubs believed to belong to Payne Stewart.

Stewart, 42, was one of golf's most recognizable players because of his traditional knickers and tam-o'-shanter hat. The Florida resident's latest victory was in June when he won his second U.S. Open title.

Competition scheduled for Friday in two PGA Tour events will be postponed because of a memorial service scheduled at the First Baptist Church of Orlando.

The Tour Championship in Houston, a $5 million event for the top 30 players on the money list, will start Thursday after a separate memorial to be held at the first tee. Afterward, competitors will play 27 holes of golf instead of the traditional round of 18.

Another 27 holes will be played on Saturday, with an 18-hole final round on Sunday.

The Southern Farm Bureau Classic at Madison, Mississippi, will be just 54 holes long, with rounds scheduled for Thursday, Saturday and Sunday.